This invention relates generally to the field of floating offshore platforms or vessels for the exploitation of undersea deposits of petroleum and natural gas. More specifically, it relates to a system and apparatus for tensioning risers that extend from a subsea wellhead or subsurface structure to a floating platform or vessel.
Offshore platforms for the exploitation of undersea petroleum and natural gas deposits typically support production risers that extend to the platform from one or more wellheads or structures on the seabed. In deep water applications floating platforms (such as spars, tension leg platforms, extended draft platforms, and semi-submersible platforms) are typically used. These platforms are subject to motion due to wind, waves, and currents. Consequently, the risers employed with such platforms must be tensioned so as to permit the platform to move relative to the risers. Also, riser tension must be maintained so that the riser does not buckle under its own weight. Accordingly, the tensioning mechanism must exert a substantially continuous tension force to the riser within a well-defined range.
One broad class of risers is the category called “Top Tensioned Risers” or TTRs. Such risers extend from the subsea wellheads below the hull of the platform substantially vertically to the deck area of the platform, where they are supported by a tensioning mechanism; hence the term “Top Tensioned Riser.” Each TTR typically extends from a riser tension point up into the production deck levels of the platform with the use of a heavy wall conduit or stem joint. At the top of the conduit or stem joint is an upper riser termination where a surface wellhead and a production tree or flow control device are mounted. (Platforms with such an arrangement are called “dry tree” platforms.) A flexible jumper attached to the production tree enables the produced well fluids to be transferred to the topside processing facilities.
Passive buoyancy cans are a well-known type of riser tensioning mechanism that is used primarily on spars. The buoyancy cans independently support each TTR, which allows the platform to move up and down relative to the riser. This isolates the risers from the heave motion of the platform and eliminates any increased riser tension caused by the horizontal offset of the platform in response to the marine environment.
Hydro-pneumatic tensioner systems are another form of riser tensioning mechanism used to support TTRs on various dry tree platforms. Hydro-pneumatic riser tensioning has its origins in the support of drilling risers of MODUs (mobile offshore drilling units). A plurality of active hydraulic cylinders with pneumatic accumulators is connected between the platform and the riser to provide and maintain the necessary riser tension. Platform responses to environmental conditions, mainly heave and horizontal motions causing hull set-down, necessitate changes in riser length relative to the platform, which causes the tensioning cylinders to stroke in and out. The spring effect caused by the gas compression or expansion during riser stroke partially isolates the riser from the low heave platform motions while maintaining a nearly constant riser tension. However, when the platform takes a significant horizontal offset, the compression of the gas in the cylinders causes increased cylinder pressure and thus increased riser tension. The magnitude of this increased riser tension is a function of the stiffness of the riser and the tensioning system.
Two major types of hydro-pneumatic tensioner systems are currently in use: the “push” or compression style system and the “pull” or tension style system. Both systems use hydraulic cylinders having pistons with piston rods connected to the riser by a tension ring device. Push-style cylinders are mounted with the piston rods looking up, and they use pressure applied to the piston side of the cylinders to provide riser tension. The piston rods effectively push up on the riser, putting the rods in compression while providing the necessary riser tension. The pull-style cylinders, by contrast, are mounted with the piston rods looking down. Pressure applied to the rod side of the cylinders puts the piston rods in tension while pulling up on the riser to generate the riser tension.
Pull-style tensioner systems have to date been used predominately on tension leg platforms (TLPs) to support TTRs. The tensioner cylinders may be symmetrically mounted under the well deck, outboard of the riser, using padeyes and shackles, or they can be mounted in a similar manner in a cassette frame that is then mounted to the well deck. The cylinders are angled inboard to riser attachment points on a tension ring. Generally, a roller assembly mounted at the well deck level above the tension ring is used to provide lateral support to the riser as it passes through the tensioner.
The pull-style tensioners on TLPs are designed for short strokes due to the low heave characteristics of the hull, combined with the relatively small riser length changes associated with small hull set down due to the parallelogram arrangement formed by the platform, tendons, risers, and the seafloor well pattern. The advantage is that the surface production tree or flow control device at the top of the riser on a TLP can be mounted closer to the tensioning point of the riser, and the well spacing inside the platform can be reduced. This reduces the bending loads induced in the portion of the riser above the tension point, i.e., the upper riser stem joint, from the dynamic motions of the surface production equipment. However, the production equipment for other hull types and riser system configurations may be located some distance away from the tensioning point. Because there is generally only one set of lateral motion restraining devices (such as rollers) to restrain the riser laterally, dynamic bending moments from the production equipment are transferred across the rollers and the tension ring into the riser pipe below the tension point. Also, riser vortex induced vibration (VIV) oscillations can be transferred across the tension ring and into the upper riser stem joint, possibly affecting its fatigue life.
If a tension cylinder failure occurs, the eccentric load generated by the unequal application of cylinder forces at the tension ring may also cause additional bending moments that must be reacted to by the riser pipe. The unbalanced cylinder forces can also cause the riser and the surface tree to lean to one side. The occurrence of dynamic bending moments from the production equipment and the failed cylinder scenario dictate that the tensioning cylinders be mounted so as to allow pivoting, such as with the use of padeyes and shackles. Pivot mounting eliminates the need for the cylinders and cylinder supports to react to the various loads. However, because the cylinders are generally hung from above to pull up and are also angled inboard to the riser, failed cylinder change-out is made more difficult because of the location of the cylinders below the hang-off deck.
Push-style tensioner systems are a more recent approach to riser tensioning and have been used on deepwater spars to support TTRs and drilling risers. Typically, four to six push-style cylinders are vertically mounted to the platform deck. A piston is journaled in each of the cylinders, each of the pistons being connected to an upwardly-extending piston rod that is attached to a structural top frame. The structural top frame, in turn, supports a large diameter conductor pipe and contains the tension ring attachment to the riser. The piston rods push up on the top frame, which, in turn, pushes up on the riser via a tension ring. The conductor pipe, with two sets of reaction rollers, creates a two-point force coupling to react to riser dynamic bending moments generated from the production equipment and failed cylinder-induced bending moments. The conductor pipe and the associated anti-rotation devices also resist riser torque induced by platform or vessel yaw motions. Because the rods are in compression and are required to resist buckling under very large loads, the rod diameters are larger than those of a pull-style tensioner system.
In general, while conventional pull-style tensioners, as described above, are generally smaller, less expensive, and more widely available than push-style tensioners, the typical pull-style tensioner system generally exhibits one or more of the following disadvantages: (1) It may not provide two-point reaction to riser dynamic bending moments generated by surface production equipment located above the riser tension point. (2) The lack of two-point reaction also allows riser VIV oscillations below the tension point to excite the surface equipment above the tension point, thus adversely affecting its fatigue life. (3) It may not react adequately to failed cylinder eccentric loads, thus creating additional riser bending moments. (4) It may not sufficiently resist riser rotation (torque) created by platform yaw motions. (5) Failed cylinder replacement is made more difficult by below-deck work requirements.